The present invention pertains generally to lasers and more particularly to Raman scattering using optical waveguides. The present invention comprises an improvement over U.S. Pat. No. 4,222,011 entitled "Stokes Injected Raman Capillary Waveguide Amplifier" by Norman A. Kurnit, U.S. Pat. No. 4,224,577 entitled "A Multistaged Stokes Injected Raman Capillary Waveguide Amplifier" by Norman A. Kurnit, U.S. Pat. No. 4,194,170 entitled "Shifting of Infrafed Radiation Using Rotational Raman Resonances in Diatomic Molecular Gases" by Norman A. Kurnit, and U.S. Patent application Ser. No. 229,023 filed Jan. 27, 1981 entitled "A Ring Cavity for a Raman Capillary Waveguide Amplifier" by Norman A. Kurnit and U.S. Patent application Ser. No. 243,309 filed Mar. 13, 1981 entitled "Combination Ring Cavity and Backward Raman Waveguide Amplifier" by Norman A. Kurnit all of which are incorporated herein by reference for all that they teach. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
It is often desirable, particularly for infrared and longer waveforms, to confine radiation to a small mode volume over distances long compared to the distance for which diffraction spreading is appreciable. Hollow dielectric waveguides such as disclosed in E. A. J. Marcatili and R. A. Schmeltzer, "Hollow Metallic and Dielectric Waveguides for Long Distance Optical Transmission and Lasers," Bell Syst. Tech. J. 43, 1783 (1964), have been used for such confinement and have had numerous applications to the development of discharge-pumped lasers such as disclosed in R. L. Abrams, "Waveguide Gas Lasers," in Laser Handbook, Vol. 3, ed. by M. L. Stitch, North Holland, 1979, p. 41, and references therein. Hollow dielectric waveguides have also been used for absorption spectroscopy such as disclosed in M. A. Guerra, A. Sanchez, and A. Javan, ".nu.=2.theta.1 Absorption Spectroscopy of Vibrationally Heated NO Molecules Using Optical Pumping in a Waveguide," Phys. Rev. Lett. 38, 482 (1977), optical pumping such as disclosed in M. Yamanaka, "Optically Pumped Waveguide Lasers," J. Opt. Soc. Am. 67, 952 (1977), and stimulated Raman scattering such as disclosed in P. Rabinowitz, A. Kaldor, R. Brickman, and W. Schmidt, "Waveguide H.sub.2 Raman Laser," Appl. Op. 15, 2005 (1976), N. A. Kurnit, G. P. Arnold, L. M. Sherman, W. H. Watson, and R. G. Wenzel, "CO.sub.2 -Pumped p-H.sub.2 Rotational Raman Amplification in a Hollow Dielectric Waveguide," Conference on Lasers and Electro-optic Systems, CLEOS/ICF '80, San Diego Calif., Feb. 1980. However, very long lengths of such dielectric waveguides are cumbersome and difficult to construct particularly since tight tolerances with regard to straightness are required in order to prevent conversion into lossy higher-order modes.
Bent rectangular metallic waveguides have been studied extensively by E. Garmire, T. McMahon, and M. Bass, "Propagation of Infrared Light in Flexible Hollow Waveguides," Appl. Opt. 15, 145 (1976), E. Garmire et al., "Flexible Infrared-Transmissive Metal Waveguides," Appl. Phys. Lett. 29, 254 (1976), E. Garmire et al., "Lowloss Optical Transmission Through Bent Hollow Waveguides," Appl. Phys. Lett. 31, 92 (1977), E. Garmire et al., "Low-loss Propagation and Polarization Rotation in Twisted Infrared Metal Waveguides," Appl. Phys. Lett. 34, 35 (1979), E. Garmire et al., "Flexible Infrared Waveguides for Highpower Transmission," IEEE J. Quant, Electron., QE-16, 23 (1980), as a means of steering CO.sub.2 laser radiation for cutting, welding, and surgery. The principal disadvantage of bent rectangular metallic waveguides is that the walls perpendicular to the electric field give a relatively large attenuation coefficient. For straight metallic waveguides, it has been demonstrated, however, that the walls perpendicular to the electric field may be removed completely if the walls parallel to the electric field are given a slight curvature (.rho. ) which keeps the mode focused in the center of the guide such as disclosed by T. Nakahara and N. Kurauchi, "Guided Beam Waves Between Parallel Concave Reflectors," IEEE Trans. on Microwave Theory and Techniques, MTT-15, 66 (1967), H. Nishihara, T. Inoue, and J. Koyama, "Low-Loss Parallel-Plate Waveguide at 10.6 .mu.m," Appl. Phys. Lett. 25, 391 (1974), H. Nishihara, T. Mukai, T. Inoue, and J. Koyama, "Self-Focusing Parallel-Plate Waveguide CO.sub.2 Laser with Uniform Transverse Excitation," Appl. Phys. Lett. 29, 577 (1976). Use of such a waveguide bent in a circle or helix has been previously proposed by M. E. Marhic, L. I. Kwan, and M. Epstein, "Optical Surface Waves Along a Toroidal Metallic Guide," Appl. Phys. Lett. 33, 609 (1978), M. E. Marhic et al., "Invariant Properties of Helical-Circular Metallic Waveguides," Appl. Phys. Lett. 33, 874 (1978), M. E. Marhic et al., "WhisperingGallery CO.sub.2 Laser," IEEE J. Quantum Electron. QE-15, 478 (1979), L. W. Casperson and T. S. Garfield, "Guided Beams in Concave Metallic Waveguides," IEEE J. Quantum Electron. QE-15, 491 (1979), for flexible guiding of CO.sub.2 laser radiation and the construction of a CO.sub.2 laser. However, this is the only disclosed use of this device.